Most of us become attached to our characters at some point, some people overly so, but the attachment is greater than just getting in the character’s head and using their motivations in the game instead of our own. There comes a point when we mentally care whether or not a character lives or dies, and some attachments are way stronger than others. The level of attachment doesn’t seem to diminish with the years of experience; it seems mostly to be affected by the mentality of the player. That is what makes role-playing that much more interesting. Players will have their characters do things outside of player-knowledge, even though it will get their character killed, maimed, or have a high chance of death, because they know it is what their character would do.
Take the following example that happened at my game table. A character was possessed by a spirit and the spirit was driving them to go and retrieve a magical sword. Once the sword was retrieved, the character was going to be jealous of the others and would have it in mind that his traveling companions would be trying to take it, and that was just unacceptable. There were three characters that were possessed by the time the sword was retrieved. The rest of the party kept knocking them down, not killing them, just beating them down enough to drive the spirit out of them. By the time the sword was actually in-hand, there was a trail of PCs leading to it and then the real carnage happened.
The player that got the sword was playing it to the hilt. His character was overcome with a drive to get out of the buried temple they were in and the other PCs were trying to stop the character; mostly they were trying to stop the spirit from escaping. The escaping PC, however, took it upon himself to execute every single fallen PC in the temple as he made his way to the outside.
Almost every character in the party was killed. The last possessed character was put down and only two of six PCs made it out alive.
No one was angry. They were semi-attached to their characters, enough that they cared that the characters died, but not enough to be angry about it. Every single one of them played it to the hilt, and then they got to thinking about what their next character was going to be.